The present invention relates to a new and improved bioremediation process for anaerobic biodegradation, detoxification and transformation of organic compounds in contaminated aquifers. Bioremediation is a process of either promoting or introducing organisms, plants or other flora or fauna to break down or use the organic compounds.
During biodegradation of petroleum hydrocarbons including BTEX compounds (which include benzene, toluene, ethylbenzene and xylene isomers) in groundwater, micro-organisms use dissolved BTEX as an electron donor and rely on a variety of terminal electron accepting processes (TEAP) such as aerobic oxidation, nitrate reduction, iron (III) reduction and sulfate reduction to generate energy and produce new biomass. Aerobic oxidation provides the most potent energy source for the micro-organisms. However, dissolved oxygen solubility in water is relatively low, and in the vicinity of a BTEX source, dissolved oxygen is rapidly consumed, resulting in anaerobic groundwater conditions. Under these conditions electron accepting processes with nitrate and sulfate become more important. When all the soluble electron acceptor salts are depleted, groundwater conditions become conducive to fermentation and methane is generated (methanogenesis). A number of laboratory and field studies have demonstrated that BTEX compounds are susceptible to anaerobic biodegradation under nitrate and sulfate reducing conditions.
Conventional engineering approaches to clean-up BTEX-impacted groundwater often rely on adding oxygen to stimulate biological breakdown of dissolved BTEX. This can be very effective at sites where conditions are conducive to distribution of oxygen into the impacted groundwater. However, oxygen addition under reducing conditions is very inefficient both due to its limited water solubility and also due to a number of chemical processes that rapidly consume oxygen (e.g. oxidation of reduced minerals and other non-target compounds).
Biodegradation of BTEX by native sulfate reducing micro-organisms consumes the naturally occurring sulfate in petroleum-impacted aquifers. Also, sulfate reduction is very widespread in gasoline impacted groundwater and that many plumes, which contain dissolved organics, are depleted in sulfate. As such, there is a great potential to exploit this process and expedite clean-up of petroleum impacted groundwater if sulfate can be added to the impacted groundwater. A number of field studies have demonstrated enhanced biodegradation of dissolved BTEX following sulfate addition to groundwater. At other sites, where nitrate is the limiting electron acceptor, field scale applications using nitrate addition have been shown to be effective in reducing the BTEX concentrations in soil and groundwater.
However, addition of a limiting electron acceptor salt via injection wells or infiltration galleries has limited effectiveness in remediating contaminated groundwater. Distribution and delivery using these methods is largely controlled by site geology and is constrained by the geometry of the injection well layout. Furthermore, electron acceptor salt injection into the aquifer does not address hydrocarbon impacts in the vadose zone between the top of the ground surface and the water table. For treating larger areas, the number of wells and addition events can be cost prohibitive and the treatment may be localized in the area near the injection. In addition, the periodic injection of the electron acceptor salt may cause changes in the subsurface conditions (e.g. electrical conductivity and possibly pH) that may not be conducive to optimal biological response as the subsurface microbial populations are constantly adapting to these changes.
The need remains for a more stable, uniform and cost-effective electron acceptor salt delivery mechanism to achieve better treatment efficiency of contaminated groundwater.